The invention relates generally to a method of and a device for applying a film or a coating to a workpiece.
More particularly, the invention relates to a method of and a device for spraying a film or a coating onto a workpiece.
A known procedure for applying a film or a coating to a workpiece involves the use of a first spray consisting of pneumatically atomized material and a second spray consisting of hydrostatically atomized material. Pneumatic atomization is performed with a nozzle having a material discharge opening and an air discharge opening. Material to be atomized is delivered to the material discharge opening while air at a pressure sufficient to atomize the material, e.g., at a pressure of 1 to 10 bars, is delivered to the air discharge opening. The material is drawn through the material discharge opening due to suction created by the atomizing air or is advanced through the material discharge opening under low pressure. In hydrostatic atomization, on the other hand, material to be atomized is forced through a relatively small nozzle at such a high pressure, e.g., at a pressure of 30 to 500 bars, that the material atomizes upon issuing from the nozzle. During spraying of the workpiece, the workpiece and the sprays are moved relative to one another.
U.S. Pat. No. 3,927,833 to Harrison et al. discloses an atomizing device having three spray heads which are arranged next to one another in such a manner that the axis of each head extends at an angle to the axes of the other heads. The two outermost heads function to hydrostatically atomize a first material which is under hydraulic pressure while the central head functions to pneumatically atomize a second material by means of compressed air. The device further has discharge openings through which a particulate material and a fibrous material are fed to the sprays issuing from the heads. A common control unit is provided for the spray heads and the supply means for the particulate and fibrous materials, and the control unit has a lever which enables the supply means and all of the heads to be placed into operation simultaneously.
The device of Harrison et al. has the drawback that it yields a very non-uniform coat. The reason is that each of the three sprays entrains air from the surroundings. Therefore, air cushions are formed between neighboring sprays, and these air cushions affect the homogeneity of mixing as well as the completeness of the spray pattern. As regards mixing, symmetry exists across the central plane, and the combined sprays expand in the direction of such plane. The application of material to a workpiece during relative horizontal movement of the atomizing device and the workpiece differs significantly from that which occurs during relative vertical movement. Furthermore, the homogeneity of mixing and the completeness of the spray pattern depend in large measure upon the distance between the atomizing device and the workpiece. Moreover, the atomizing device is large and heavy. It can hardly be manipulated manually and it is difficult or impossible to introduce the spray heads into cavities.
German Patent No. 647 713 discloses an atomizing device in the form of a spray gun which is capable of producing a spray by pneumatic atomization only. The spray head of this device has a pneumatic atomizing arrangement including a material discharge nozzle which is supplied with a material to be atomized at low pressure, and an air discharge nozzle which is supplied with at least one current of atomizing air at a pressure sufficient to atomize the material. The spray head is further provided with outlet openings for the discharge of additional currents of air into the spray. The device includes a control unit with a manually operable lever which controls valves for the air and the material to be atomized. Air flows to the air discharge nozzle and the outlet openings through a common air valve. An adjustable throttling mechanism makes it possible to vary the ratio of atomizing air to additional air. The pneumatic atomization results in a spray which consists of fine droplets and can produce a film of only limited thickness. If it is attempted to increase the film thickness by increasing the throughput of material to be atomized, atomization worsens considerably. While it is possible to reduce or avoid worsened atomization by increasing the air pressure and thereby increasing the throughput of air this causes disturbances due to the creation of a dense mist.
French Patent No. 2.127.874 discloses an atomizing device in which atomization is effected purely hydrostatically. This device is capable of producing large film thicknesses in a single operation. However, poor overlap is obtained due to the well-defined character of the spray. Since the conditions of use as well as the types of coating material are essentially determined by the nozzle geometry, there is little flexibility in regard to the operating conditions. Regulation of the quantity of material during application is not possible. If it is attempted to reduce the throughput of material by employing a smaller nozzle, blockages occur. On the other hand, it it is attempted to reduce the throughput by reducing the pressure on the material, the droplet size of the spray increases.
Material subjected to hydrostatic atomization may be exposed to one or more air currents which serve to assist in spray formation or to effect a slight secondary atomization. While this permits the droplet size to be increased, the drawbacks associated with pure hydrostatic atomization basically remain. A fine, uniform film such as obtained with pneumatic atomization is not achieved.
U.S. Pat. No. 3,599,038 to Skidmore discloses an atomizing device equipped with an electrostatic charging unit. Here, the workpiece to be coated is grounded, and the droplets of the spray are electrostatically charged by a high voltage electrode before travelling to the workpiece. This results in a shrouding of the workpiece which improves the coating efficiency, particularly for filigree articles. In pheumatic-electrostatic atomization, the kinetic energy of the samll droplets is reduced in order to improve the electrostatic effect. This, however, causes poorer penetration of the droplets into the recesses of a workpiece (Faraday Effect), e.g., into the spaces between the cooling ribs of a motor casing, and also causes excessive coating of the edges. In hydrostatic-electrostatic atomization carried out at higher pressures, the droplets likewise have a kinetic energy which is excessive from the electrostatic point of view although favorable as regards penetration of the droplets into recesses. If the pressure on the material to be atomized is reduced so as to achieve better utilization of the electrostatic effect, the size of the droplets increases.